A variety of linear transmitters implemented using feedback loops around a power amplifier (PA) are in use today. Linear transmitters such as Cartesian feedback transmitters, adaptive predistortion transmitters, and envelope elimination and restoration (EER) transmitters place the PA in a feedback loop in order to reduce, if not cancel, the PA nonlinearities. In such transmitters, the load of an antenna coupled to the PA changes when the antenna is in close proximity to reflective or absorptive objects. It is known in the art to use an isolator between the PA and the antenna to minimize the effect of such load changes on the PA. The weight and size of isolators, however, significantly limit their desirability in today's smaller portable communication devices (e.g., cellular phones). U.S. Pat. No. 5,675,286 discloses the use of isolator elimination (IE) circuitry that continuously tracks and corrects gain, phase, and level set changes in such transmitter feedback loops, thereby eliminating the need for isolators.
IE circuitry optimizes PA efficiency over approximately 30% of the complex
impedance plane. When the antenna environment moves the impedance (i.e., the PA load) outside the optimized region, the PA has a higher compression point. Better efficiency in these regions could be obtained by simply increasing the PA output power, but product specifications and government regulations (e.g., Federal Communications Commission and European Telecommunications Standardization Institute regulations) limit transmission power. Thus, outside the optimized region, the PA efficiency drops by approximately 10%. In a portable communication device, such a drop in PA efficiency drains the battery more quickly and results in less talk-time per battery charge. Improving the PA efficiency in such instances would have the effect of increasing battery life, and therefore, talk time.
Circuits which move the magnitude and phase of a PA load to a location where the PA has better efficiency are generally know in the art as load pull circuits. Load pull circuits must provide a means for load detection and a means for load correction. The load detection circuits detect the forward and reverse currents and voltages between the PA and the antenna and calculate the load magnitude and phase. The load detection circuits then use these calculations to drive a load adjust circuit. In portable communication devices, such load detection circuits require high frequency RF circuitry that increases the size, weight, and cost of the devices. Improving PA efficiency without incurring the costs associated with RF circuitry is clearly desirable.
Thus, there is a need for an apparatus and method for controlling the load of a PA, to improve PA efficiency in linear transmitters with IE circuitry, that does not require the use of high frequency RF circuitry.